Alkaline Membrane Fuel Cells and Apparatus and Methods for Supplying Water Thereto

ABSTRACT

A device to produce electricity by a chemical reaction without the addition of liquid electrolyte comprises an anode electrode, a polymer membrane electrolyte fabricated to conduct hydroxyl (OH—) ions, the membrane being in physical contact with the anode electrode on a first side of the membrane, and a cathode electrode in physical contact with a second side of the membrane. The anode electrode and cathode electrode contain catalysts, and the catalysts are constructed substantially entirely from non-precious metal catalysts. Water may be transferred to the cathode side of the membrane from an external source of water.

RELATED APPLICATIONS

This application is a division of and claims the benefit of U.S.application Ser. No. 13/020,614, filed Feb. 3, 2011, which is acontinuation of U.S. application Ser. No. 12/477,669, filed Jun. 3,2009, now U.S. Pat. No. 7,943,258, which claims priority to ProvisionalApplication Ser. No. 61/204,067 filed Dec. 31, 2008, as well as toProvisional Application Ser. No. 61/058,607, filed Jun. 4, 2008, each ofwhich is incorporated by reference herein in its entirety.

TECHNICAL FIELD

Alkaline membrane fuel cells (“AMFCs”) are a lower-cost alternative toproton-exchange membranes (“PEM”) fuel cells which are dependent on theuse of expensive precious metals. AMFCs eliminate the use of suchprecious metals, thereby reducing costs of manufacture. AMFCs should bedesigned with an effective supply of water to the cathode side of thefuel cell to ensure a state of hydration independent of cell currentstate, but require highly effective membrane/electrode assemblies toachieve commercial level performance with no added liquid electrolyte. Afuel cell preconditioning or pretreatment method hydrates ionomercomponents to achieve a required or desired degree of ionicconductivity. Also, liquid water may be transferred to the cathode sideof the membrane from an external source of water.

BACKGROUND

Fuel cells have generated a lot of attention recently due to increaseddemands for efficient and clean electricity generation. A majorinhibitor to mass commercialization of fuel cells is cost. AMFCs offer apromising solution to reduce the cost of effective fuel cells. Alkalinemembrane fuel cells are similar to PEM fuel cells but the membranes aredesigned to transport hydroxide ions instead of protons. One of thebiggest cost factors of PEM fuel cells is their dependency on a preciousmetal such as platinum, which is used as both the air cathode andhydrogen anode catalyst. Platinum is an extremely rare metal, occurringas only 0.003 ppb in the Earth's crust, and is 30 times rarer than gold.Reducing the amount of platinum required (and thus cost) has been amajor focus of PEM fuel cell research. Alkaline membrane fuel cellsallow elimination of Pt altogether from the fuel cell catalyst, relyingon the use of non-precious metal catalysts, which can greatly reduce thecost of fuel cells. The milder electrochemical environment of thealkaline membrane, that allows replacing precious metal catalysts bymuch less expensive metal catalysts, also allows using stack hardware ofmuch lower cost and superior properties.

Several reports on building and testing of AMFCs on a laboratory scale,have described cell testing with aqueous electrolyte, typically aqueousKOH, continuously added as part of the fuel feed stream, found necessaryto achieve reasonable performance. AMFC Performance without any addedaqueous electrolyte has been found to be at least an order of magnitudelower than that obtained with proton conducting membrane fuel cellswhich, as a rule, do not use any added electrolyte. Once liquid alkalineelectrolyte is added, a polymer electrolyte fuel cell loses some keyadvantages of this family of cells, including the avoidance ofelectrolyte management issues and maintenance of a safe exhaust streamwhich consists of water vapor alone in the case of hydrogen fuel. Aslong as it is a prerequisite for obtaining acceptable AMFC performance,continuous liquid alkaline electrolyte addition seriously diminishes thepractical value of AMFCs.

There is a significant challenge, however, to the development ofoptimized electrodes and membrane-electrode assemblies (“MEAs”)specifically designed for AMFCs which do not use any added liquidelectrolyte. The challenge has primarily to do with the limitedconductivity of OH— ion conducting polymers demonstrated to date,requiring judicious choice of recast ionomer material for the catalyst“ink” and a catalyst layer thickness and structure optimized for minimaltransport limitations at the highest demand current.

When looking at the much more developed technology of PEM fuel cells aspossible source of information, it is very important to recognize somesubstantial differences between the technology based on alkalinemembrane and the technology based on an acidic membrane. Not only is theionic conductivity of the alkaline ionomer substantially lower, thesides of the cell where water is generated and consumed are reversed. Inthe AMFC, water is generated on the fuel side and consumed on theoxygen, or air side. This creates AMFC technical challenges of specialnature that are not shared with the PEM counterpart. Product waterremoval without loss of fuel becomes an important issue as result of thewater generation at the fuel electrode. Furthermore, cathode dry out,resulting in strong cathode performance loss, is a clear danger in theAMFC, because water is being consumed at the cathode, rather thangenerated there as in the case of the PEM cell, and the active flow ofair by the cathode would tend to carry with it any cathode water out thecathode exhaust. Water management therefore has a different problem setand a higher degree of severity in the AMFC.

Fuel cells are often operated with pure hydrogen gas as the fuel. Whenusing pure hydrogen as a fuel, a PEM fuel cell can be operated in whatis known as a “dead-end” anode configuration, which means the anodecompartment has an inlet, but no outlet. Design of a fuel cell based ona dead-ended anode, has been recognized as having valuable advantages,including system simplicity, zero fuel emissions and high fuelutilization. However, in an AMFC, water is a product that is generatedin the anode side of the cell. Consequently, a dead-ended anode in anAMFC requires that there be a way to continuously remove excess productwater from the anode. Product water removal from an operating AMFCthrough an open-ended anode could mimic the established mode of waterproduct removal from fuel cells based on proton conducting membranes,where water is generated at the air cathode and leaves through theexhaust of the open-ended cathode. Such straightforward solution forexcess water removal will, however, result, in the case of the AMFC, insignificant fuel loss at high hydrogen flow rates, or poor utilizationof active electrode area when the rate of fuel supply is loweredsufficiently to avoid fuel loss. In summary, AMFC area power densitiessignificantly above 100 mW/cm², while using non-Pt catalysts andavoiding added liquid electrolyte, has not been described to date. Also,effective AMFC water management in general, and particularly watermanagement without fuel loss, the case of an AMFC configured with adead-ended anode, have not been described to date.

SUMMARY

Various aspects of the invention may provide one or more of thefollowing capabilities. An AMFC that does not require the use of liquidelectrolyte may be provided with commercially acceptable power densityand conversion efficiency. Effective hydration of a liquidelectrolyte-free AMFC as required to achieve high power density and highconversion efficiency, can be established by means of properly selectedmembrane and electrodes and by cell pre-treatment based on gradualincrease of cell current up to around 1 A/cm². Membrane electrodeassemblies which allow commercially acceptable AMFC performance withoutany added electrolyte may be provided through design and fabricationbased on proper combination of recast OH— conducting ionomer andnon-precious metal catalysts. An AMFC may be provided which operateswith hydrogen fuel using a dead ended anode by water management whichpasses water generated on the fuel anode side to the cathode side tosecure cell water release through the cathode. This configuration allowsthe AMFC to achieve high fuel utilization with a simple systemconfiguration. Alternatively, water may be transferred to the cathodeside of the fuel cell membrane from an external source of water.

These and other capabilities of the invention, along with the inventionitself, will be more fully understood after a review of the followingfigures, detailed description, and claims.

In accordance with one aspect of the present invention, there isprovided a fuel cell assembly having an anode electrode, a polymermembrane electrolyte configured to conduct hydroxyl (OH—) ions, in whichthe membrane is in physical contact with the anode electrode on a firstside of the membrane. The cell also has a cathode electrode which is incontact with the second, opposite side of the membrane. Both electrodescontain catalysts and these catalysts are constructed substantially fromnon-precious materials.

In accordance with a further aspect of the present invention, there isprovided an AMFC assembly having anode and cathode electrodes, as wellas a polymer membrane electrolyte which conducts hydroxyl ions (OH—)which is in physical contact with the anode and cathode electrodes oneach of its sides. Further, an external source of water transfers waterto the cathode side of the polymer membrane. The anode and cathodeelectrodes each contain catalysts formed substantially of non-preciousmetals.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, described below, like reference charactersrefer to the same or similar parts throughout the different views. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating particular principles of the methods and apparatuscharacterized in the Detailed Description.

FIG. 1 is a schematic illustration of an alkaline membrane fuel cellapparatus with open ended flow on both hydrogen fuel and air sides.

FIG. 2 is a schematic illustration of an alkaline membrane fuel cellapparatus configured for dead-ended anode operation.

FIG. 3 is a schematic illustration of some of the components of theanode electrode of an alkaline membrane fuel cell with a dead-endedanode.

FIG. 4 is a schematic illustration of some of the components of thecathode electrode of an alkaline membrane fuel cell with a dead-endedanode.

FIG. 5 is a schematic illustration of an alkaline membrane fuel cellwith a water exchanger apparatus on the cathode air exhaust.

FIG. 6 is a schematic illustration of the targeted water vapor pressurevariation from the anode to the cathode.

FIG. 7 is a schematic illustration of an alkaline membrane fuel cellapparatus according to the invention in a non-forced air cathodeconfiguration.

FIG. 8 is a schematic drawing illustrating a sectioned side view of afuel cell having an alkaline polymer membrane including a micro fibermesh according to one aspect of the invention.

FIG. 9 is a schematic drawing of a hydrophilic micro-fiber mesh used inthe fuel cell shown in FIG. 8.

FIG. 10 is a schematic drawing illustrating a sectioned side view of afuel cell having an alkaline polymer membrane including an anode wick, acathode wick, and a water-permeable film around the membrane accordingto another aspect of the invention.

FIG. 11 is a schematic drawing illustrating a sectioned side view of afuel cell including a current collection and gas supply plate accordingto another aspect of the invention.

FIG. 12 is a schematic top view of the plate shown in FIG. 11.

FIGS. 13A and 13B are graphs displaying fuel cell performance resultswith and without a supply of water to the cathode side of the cell,

FIG. 14 is a block flow diagram of a method of pretreatment andcontinuous active hydration of a fuel cell according to another aspectof the invention.

FIG. 15 is a graph displaying fuel cell performance in accordance withthe disclosure of Example 1 herein.

DETAILED DESCRIPTION

In describing embodiments of the invention, specific terminology is usedfor the sake of clarity. For purposes of description, each specific termis intended to at least include all technical and functional equivalentsthat operate in a similar manner to accomplish a similar purpose.Additionally, in some instances where a particular embodiment of theinvention includes a plurality of system elements or method steps, thoseelements or steps may be replaced with a single element or step;likewise, a single element or step may be replaced with a plurality ofelements or steps that serve the same purpose. Moreover, while thisinvention has been shown and described with references to particularembodiments thereof, those skilled in the art will understand thatvarious other changes in form and details may be made therein withoutdeparting from the scope of the invention. The implementations set forthin the following description do not represent all implementationsconsistent with the subject matter described herein. Instead, they aremerely some examples consistent with certain aspects related to thedescribed subject matter.

In some implementations, an alkaline membrane fuel cell is provided thatallows dead ended anode operation. The dead ended anode operationenables a simple method of fuel control to the fuel cell along with highfuel utilization. Some embodiments may include an alkaline membrane fuelcell which provides improved power density through the use of animproved method of MEA preparation with an ionomer solution specificallydeveloped for alkaline membrane fuel cell electrode preparation. Someembodiments include an AMFC that does not utilize precious metalcatalysts. Some embodiments include an AMFC that does not utilize liquidelectrolyte addition.

An ionomer is a polymeric electrolyte that comprises a poly-hydrocarbon(aromatic and non-aromatic hydrocarbons), or poly-perfluorocarbonbackbone and a fraction of ion carrying units (usually no more than15%). To incorporate the ion conducting polymer into the structure of afuel cell electrode, a solution of the corresponding ionomer is added tothe catalyst when the active layer of the electrode is prepared. Inaddition, additives for catalyst suspension stability and/or forviscosity adjustment can be added. Following a curing, or cross-linkingstep, or/and further chemical treatment to increase ionic conductivity,the ionomer in the electrode converts to a polymer resembling themembrane polymer.

Referring to FIG. 1, a generic alkaline membrane fuel cell 100 includesa fuel anode 110, an air cathode 112, an anode bipolar plate 114, acathode bipolar plate 116, an anode gas diffusion layer GDL 118, acathode gas diffusion layer GDL 120, an anode catalyst layer electrode122, a cathode catalyst layer electrode 124, and an alkaline membrane126. While a single cell 100 is shown in FIG. 1, a plurality of cellsmay be configured together to form a fuel cell stack. In the case of theAMFC, metal bipolar plates made of stainless steel or of nickel-coatedaluminum, can provide a compact and robust stack hardware of lowcorrosion susceptibility. The fuel cell consumes hydrogen as a fuel toproduce water and electrical energy. In an alkaline membrane fuel cell100, hydroxyl (OH—) ions are transported through the membrane 126, tothe anode to react with the hydrogen which produces water in the anode.At the cathode, oxygen reacts with electrons passing through an externalelectrical circuit connecting the anode to the cathode and with water toform “new” OH— ions.

The anode bipolar plate 114, both serves as a current collector for theanode of the cell and defines hydrogen fuel flow channels. The cathodebipolar plate 116, serves as both a current collector for the cathode ofthe cell and defines oxygen or air flow channels. The benign chemicalnature of the alkaline membrane and the very low probability that the pHof the water in the cell will go under pH=5.8, opens the door to the useof unique metal hardware for AMFC bipolar plates, that cannot be usedfor PEM fuel cell stacks. Stack hardware made of aluminum has strongadvantages of light weight and of high thermal conductivity whichenables effective heat removal from the stack. It does require, however,proper surface treatment to render the surface high electronicconductivity and thereby strongly lower the high contact resistanceencountered on contacting an aluminum plate to another electronicallyconducting plate by mechanical pressure only. An effective solution wasprovided here by electroless coating of 25 micrometer thick film ofnickel metal onto the machined, aluminum bipolar plates, employingcommercially available and easily formed aluminum plate material, forexample type 6061. Such coating proved highly efficient in having acontact resistances 5-10 times lower than the cell membrane resistanceand proved stable under AMFC operating conditions on both the anode andcathode sides. The anode and cathode gas diffusion layers 118, 120evenly disperse the respective gases from the flow channels over theentire face of the catalyst layer 122, 124 and forms an electricalconnection between the catalyst layer 122, 124 and the respectivebipolar plate 114, 116. The alkaline membrane 126, is specificallydesigned for the alkaline membrane fuel cell 100 and is configured forOH— ion conductivity and proper water management. When viewed as anassembled unit, the anode catalyst layer 122 & anode GDL 118, alkalinemembrane 126, and cathode catalyst layer electrode 124 & cathode GDL120, are often referred to as a S-layer Membrane Electrode Assembly(MEA). Some of the embodiments of alkaline membrane fuel cells describedherein utilize non-precious metal catalysts for the electrode catalystlayers 122, 124. Some examples of non-precious metal catalysts couldinclude silver or cobalt on the cathode and nickel on the anode. Theseexamples are provided for reference only and should not be interpretedas the only types of potential non precious metal catalysts which can beused in conjunction with the systems and methods described herein.

Alkaline membrane fuel cell membrane/electrode assembly (MEA)fabrication may be pursued using several methods: a first method basedon using a mixture of metal catalyst and solubilized OH— ion conductingpolymer (a catalyst “ink”) and/or a second method based on a layer ofthe catalyst deposited onto the membrane surface, with or withoutpost-impregnation by recast polymer and/or a third method where anelectrode with a catalyst layer prefabricated using another,non-conducting bonding agent, is post-impregnated by the ionomer.

MEA fabrication for AMFCs based on the application of a catalyst ink,has been implemented with the dispersion of catalyst particles in asolution containing a dissolved anion conducting ionomer in anappropriate solvent with optional viscosity adjusted by additives, e.g.,glycerol, and optional suspension stabilizing additives, e.g., ethyleneglycol. In some cases, a mixture of anion exchange ionomers is used tooptimize viscosity and ionic properties of the ink. Catalysts for thisanion conductive inks include Cobalt based metal particles, Silver basedmetal particles, and other non-precious metal catalysts, either alone ordeposited onto high surface carbon structures.

Following vigorous stirring, the resulting catalyst ink may be sprayedonto a decal sheet made of Teflon and, after drying, may be hot pressedonto the membrane in chloride, or bromide form at temperatures around100 degrees C. for about 1 to 3 minutes. Alternatively, spraying orbrushing of the ink may be performed directly onto the GDL, or themembrane. Screen print and/or a tape casting methods can also be used todeposit the ink onto a Teflon sheet, the GDL, or the membrane. A fivelayer MEA may be formed by adding carbon cloth GDL layers on both sidesof a catalyzed membrane and hot pressing under similar conditions asabove. The MEA may be next ion-exchanged in 1M (molar) NaOH or KOHsolution to bring the ionomer to the OH— form and is then washed withwater and inserted in the cell, or stack. Alternatively, the membrane isconverted to OH— form before the electrodes are hot-pressed onto it.

For fabrication of AMFC MEAs based on ionomer impregnation of gasdiffusion electrodes (GDEs), commercially available phosphoric acid fuelcell type GDEs (E-Tek division of BASF) may be utilized. These GDEs havea catalyst layer in front of a GDL, with the catalyst particles bondedby PTFE. To impregnate such catalyst layer by OH— conducting ionomer, asolution of the ionomer in an appropriate solvent may be, for instance,sprayed so as to infiltrate the porous layer. Following solventevaporation, an ionomer network is generated within the catalyst layerproviding effective ionic access to the catalyst particles.

Another preferred mode for optimizing conductance and stability of theionomer in the electrode, is to use the polymer in its precursorchloromethyl form, in the preparation of the ink, apply the ink onto aGDL and next immerse the electrode built with the precursor polymer in aconcentrated solution of a mono-amine or a poly-amine, or a mixture ofmono- and poly-amines. In this way the conductivity/stability propertiesof the ionomer in the electrode is optimized after the recasting step,through addition of ion-conducting groups and introducing cross-linkingLarger amines are preferred in order to increase stabilization bycross-linking of the recast polymer in the environment of fuel cells.After aminating, the ionomer in the electrode needs to be ion exchangedwith NaOH or KOH solution, washed with water, and pressed onto themembrane, as described before.

The last procedure can be also used when infiltrating ionomer inprecursor, chloromethyl form into a pre-fabricated electrodes, followedby the amine(s) treatment described.

Design of an MEA for a polymer electrolyte fuel cell which is challengedby relatively low ionic conductivity, requires new solutions, differentfrom those provided by the extensive work on proton conducting membranefuel cells. In such past work, the catalyst layer has been typicallyconstructed of 20% volume fraction each of carbon-supported Pt catalystand of recast ionomer, with 60% void volume maintained enablingsufficient diffusivity of gaseous reactants and products. The overallthickness of the catalyst layer in proton conducting membrane fuelcells, has been typically 5-20 μm. In the case of the AMFC, as long asthe conductivity of the OH— ion conducting ionomer within the catalystlayer is only 30%-50% that of the proton conducting counterpart, theeffective thickness of the catalyst layer that enjoys good ionic accesswill be likely not more than 5 μm and possibly less. In a hydrogen/airAMFC, the important impact of such limited access of ions to/fromcatalyst sites, is on the performance of the air electrode. With theoxygen reduction cathode process being intrinsically slow, a largernumber of active catalyst sites per unit membrane area is vital forachieving higher air cathode performance and, consequently, higher cellperformance. There are several possible ways to maximize catalyst layerperformance under given limits of AMFC ionomer conductivity. Accordingto one such approach, the ionomer used for the AMFC catalyst layerfabrication, may be chosen to have significantly higher ionicconcentration vs. that of the ionomer used for membrane fabrication.Enhancing the conductivity of the membrane itself by increasing thevolume concentration of ions, is limited by mechanical integritydemands. At ionic concentrations that are too high, the membrane tendsto swell significantly when fully hydrated and, consequently, themembrane material loses mechanical integrity, particularly so followingrepeated membrane hydration/dehydration cycles. In contrast, the ionomerwithin the catalyst layer structure is supported on the solid catalystand faces somewhat lower mechanical integrity demands. It could,therefore, be prepared with higher OH— ion concentration, therebyclosing some of the specific conductivity gap between protonic and OH—ion conducting ionomers. Moreover, with the cathode in the AMFCconsuming water (rather than generating water), an ionomer of higher ionconcentration recast within the cathode catalyst structure, isbeneficial in preserving acceptable conductivity levels at lower wateractivities. Also, the risk of ionomer over-swelling is less of an issuein the relatively dry AMFC cathode.

A second approach to enhancing MEA performance when ionomerconductivities are lower, is by optimized placement of the catalyst.Given the limited access of ions to catalyst sites several micrometersaway from the membrane surface, placement of the highest possiblepopulation of active catalyst sites nearest the membrane surface, has tobe a leading principle in catalyst layer design. One way to pursueoptimized catalyst placement/distribution, is by building in adistinguishable way the 1-2 μm of the catalyst layer immediatelyadjacent the membrane surface. Such a catalyst layer “front” is to be asheavy as possible in active metal surface area, i.e., heavier than thenormal set in proton conducting polymer electrolyte cells. Additionally,the mode of catalyst application for such “front” of the catalyst layer,could be quite different than the catalyst ink application routinecommonly used in PEM cells. One possible such mode, is to use an arrayof micro-dendrites, sputter coated with the selected metal catalyst andimplanted into the membrane surface, along the lines of a technologydeveloped to date for Pt catalysts and proton conducting membranes. Theother option is to sputter nanometer sized metal catalyst nano-patches,or nano-grains onto the membrane surface, in lieu of, or in addition tothe implanted, catalyst coated dendrite array. Such approaches achieveplacement of maximum catalyst within about 1 μm from the membranesurface—a key for achieving maximum catalyst utilization undercircumstances of limited ionomer conductivity.

As to the unique challenge of proper water release from the AMFC, anoption of operation with a dead-ended anode actually exists for ahydrogen/air AMFC, in spite of the apparent conflict with the need toremove product water from the anode side of the cell. The insightrequired for realizing this option, is that dead ended anode operationcan be sustained without build-up of over-pressure when using neat(100%) hydrogen feed and water the only anode product. Under suchconditions, pressure buildup in the dead-ended anode beyond that of thehydrogen gas which is controlled by an upstream valve, is the limitedpressure of saturated water vapor pressure at the cell temperature. Theremoval of AMFC water product in operation with a completely dead-endedanode is described herein as part of a novel AMFC platform taught.

Effective water transport across the membrane from a dead-ended side ofthe cell where water is being generated, to the other side of the cellwhere excess water is to be exhausted, is a novel idea which isapplicable specifically to the alkaline membrane fuel cell. Watertransport rate through alkaline membranes is sufficient for suchpurpose, based on their ability to pass water fluxes corresponding to 1A/cm² following an initial cell pre-treatment step described herein.

FIG. 2 provides a scheme of an AMFC with dead-ended anode, showing thepattern of water flow and the mode of hydrogen entry into the dead-endedanode chamber. Referring to FIG. 2, an AMFC 200 configured fordead-ended anode operation includes a cathode chamber 210, a cathodeelectrode 212, an OH— conducting membrane 214, an anode electrode 216, adead-ended anode chamber 218, and a check valve 220. The anode andcathode electrodes 216, 212 along with the membrane 214 are configuredto achieve sufficient water transport rate from the anode chamber 218,through the membrane 214 to the cathode chamber 210 to facilitate excesswater removal from the dead-ended anode and provide effective watersupply to the water consuming cathode of the AMFC. Periodical briefpurging of excess water out the anode chamber may be required and, inthis case, a one-way valve may be added opening 218 to the ambient bybrief application of anode overpressure.

To further facilitate water transport from anode to cathode compartmentthrough the AMFC membrane, properties of the backing, gas diffusionlayers, can be tailored to achieve water collection into the membrane onthe anode side and controlled rate of water removal on the cathode side.Penetration of water through the skin of an ionomeric membrane isfacilitated when the water is presented to the surface in liquid formand the surface is, consequently, well hydrated and swollen. Toencourage water condensation on the surface of the membrane facing thedead-ended anode chamber where water product collects, the anode backinglayer (GDL) can be rendered partly hydrophilic. This can be done, forexample, by interweaving hydrophilic fibers with the wet-proofed carbonfibers to form the anode backing layer, thereby forming a backing layercross-sectional area in contact with the membrane surface which enables,on part of it, condensation of water along hydrophilic fibers and, onthe rest of it, diffusion of hydrogen through pores with hydrophobicwalls, defined by the wet-proofed carbon fibers.

FIG. 3 describes schematically the components of the anode electrode foran AMFC operating with a dead-ended anode and zero, or minimal supply ofwater from an external source. Referring to FIG. 3 with furtherreference to FIG. 2, the anode electrode 216 includes as shown in theclose up view 300, a catalyst layer front 310, a remainder of thecatalyst layer 312, and an anode gas diffusion backing layer 314. Asdescribed above the catalyst layer front 310 is immediately adjacent tothe membrane surface 214. The inclusion of a catalyst layer front 310enhances and optimizes the MEA performance through the placement of ahigh concentration of active catalyst sites closest to the membranesurface 214. In various embodiments, the catalyst layer front 310 may beapplied by sputtering, e-beam or electroless deposition. The rest of thecatalyst layer 312, is behind the catalyst layer front 310 and isadjacent to the anode gas diffusion backing layer 314. In someembodiments, the anode gas diffusion backing layer 314 may beinterweaved with a hydrophilic fiber as described above which acts as asponge to facilitate water transport from the anode 218, through themembrane 214 to the cathode 210, by presenting the membrane surface withwater in liquid form that has been produced in the anode 218, even whenthe water collected in 218 is all in mostly in vapor form.

AMFC cathode parameters that need optimization to ensure water releaseat a desirable rate, depend on whether cell operation is withactive/forced air supply to the cathode or by natural convection “airbreathing” alone. In the case of active air flow, one important issue isto avoid cathode dry-out by entry of dry air which then collects watermoisture after entry and sweeps it out while exiting the stack. Thisissue has been addressed in the case of proton conducting polymerelectrolyte fuel cells, by using various types of water exchangersupstream of the cathode inlet, that allow dry air entering the cell toextract moisture from the exhaust air stream. Various designs of waterexchangers may be incorporated into the overall water management systemof the AMFC to ensure that the cell operates on a “water neutral” basis.The term “water neutral” means that the cell or stack of cells generatesall of its required water internally without the need for an externalsupply of water. In the specific case of an AMFC, when operating with anopen-ended anode a water exchanger can be used to pass on water to theincoming air stream not only from the cathode exhaust but also from theanode exhaust, as explained below for “mode 2” type humidification ofAMFCs.

Another tool to control the rate of water vapor swept out the cell withthe air stream, is a highly hydrophobic micro-porous layer made of PTFEbonded carbon powder, placed between the cathode catalyst layer and thecathode backing layer (cathode GDL). Such micro-porous layer is taskedhere with lowering the rate of release of water to the air stream.Together with facilitated incorporation of product water into the anodeside of the membrane, it helps confine enough water within the MEAduring cell operation.

FIG. 4 describes schematically the components of the cathode electrodefor an AMFC operating with zero, or minimal supply of water from anexternal source. Referring to FIG. 4 with further reference to FIG. 2,the cathode electrode 212 includes as shown in the close up view 400, acatalyst layer front 410, a remainder of the catalyst layer 412, ahydrophobic microporous layer 414, and a cathode gas diffusion backinglayer 416. As described above the catalyst layer front 410 isimmediately adjacent to the membrane surface 214. The inclusion of acatalyst layer front 410 enhances and optimizes the MEA performancethrough the placement of a high concentration of active catalyst sitesclosest to the membrane surface 214. The rest of the catalyst layer 412is behind the catalyst layer front 410 and adjacent to the hydrophobicmicroporous layer 414. The hydrophobic microporous layer 414 repelsliquid water and does not allow it to cross out to the diffusion backinglayer 416. The hydrophobic layer thus acts as a “water dam,” loweringthe rate of water escape out the cathode catalyst layer and subsequentloss into the cathode air stream. Some of the embodiments of alkalinemembrane fuel cells described herein utilize non-precious metalcatalysts for the electrode catalyst layers 310, 312, 410, and 412. Someexamples of non-precious metal catalysts could include silver or cobalton the cathode and nickel on the anode. These examples are provided forreference only and should not be interpreted as the only types ofpotential non precious metal catalysts which can be used in conjunctionwith the systems and methods described herein.

FIG. 5 describes the option of adding a water exchanger apparatus whichenables incoming cathode air to collect moisture from the cathode airexhaust stream. Referring to FIG. 5, an embodiment of an AMFC 500,includes a water exchanger 510 configured and disposed to exchange waterfrom the cathode exhaust stream to the cathode inlet stream. Thisexchanger device 510 may be used to humidify the inlet cathode air 514with moisture that has been generated by the fuel cell and wouldotherwise be lost with the cathode exhaust 516. 512 is a membrane ofhigh water permeability and low gas permeability enabling effectivetransport of water from the humidified exhaust stream to an inlet stream516 of dry air 518, while preventing any mixing of inlet and exhaustair.

FIG. 6 depicts the nature of the water profile targeted along thethickness dimension from anode to cathode of an operating AMFC usingzero, or a very low rate of external water supply. One fundamentalrequirement is to maintain a water gradient along the cell thickness(with a higher relative humidity on the anode and a lower humidity orwater content in the cathode) in order to transport excess water productin an operating cell from the dead-ended anode to the cathode. Once theexcess water has transported to the cathode, it may be removed out ofthe cell with the cathode air exhaust stream in liquid and/or vaporform. At the same time, however, a second fundamental requirement isthat such a water gradient exists under cell operation conditions whilemaintaining relatively high water content all the way between the anodecatalyst layer and the cathode catalyst layer. This ensures propermembrane hydration and consequently maintenance of high ionicconductivity in the membrane and within both electrodes, as required toachieve acceptable cell performance. FIG. 6 shows that the level ofwater activity in the anode catalyst layer corresponds to“P*_(H20,TCELL)”, the saturated water pressure at Tcell, the temperatureof the cell. The water loss out of the MEA and into the air stream maybe kept at a sufficiently moderate level through the use of a cathodebacking layer (GDL) of sufficiently thick dimensions and sufficientlevel of wet proofing, combined with pre-humidification of the incomingcathode air stream with a water exchanger as described above in FIG. 5.The water activity at the outer edge of the cathode gas diffusion layer416 (adjacent to the cathode flow field 210) can be maintained withthese combined cell and system elements, at a significant fraction(>50%) of P*_(H20,TCELL), thereby ensuring, at the same time, a highwater level throughout the MEA and a sufficient gradient between anodeand cathode to release excess water.

The concept of “water activity” comes from physical chemistry. It is aterm covering the “escaping tendency,” or (Gibbs) free energy ofdifferent states of water, including both vapor, where the vaporpressure is the obvious measure of free energy, and water incorporatedin a polymer membrane. The water activity of water in the membrane, hasa certain energy state, or escaping tendency (“activity”) which ismeasurable by the water vapor pressure in equilibrium with the membranecontaining that specific amount of water.

Effective water management in an AMFC operating in a dead-ended anodeconfiguration enables commercially significant performance to beobtained under conditions of very high fuel utilization. The watertransport mechanics of FIG. 6 are shown by following the path of waterfrom generation in the anode 218 to removal or exhaust from the cellcathode 210. First, water is generated via the chemical reaction ofcombining hydrogen molecules and hydroxyl (OH—) ions at the face of themembrane 214 adjacent to the anode catalyst 216. Some of the waterevaporates until the partial pressure of water vapor equals thesaturated vapor pressure at the temperature of the cell. If, as in theexemplary embodiment, the anode is a sealed volume due to dead-end anodeconfiguration, and water generation rate is constant, liquid water willstart to build up unless water can diffuse through the membrane 214 at arate equal to that of water generation. Water diffuses through themembrane 214 from the anode 218 to the cathode 210 as a function of thewater activity difference between the cathode and the anode. Oncereaching the cathode catalyst/membrane interface, water is partiallyconsumed as a reactant. The liquid water at the surface of the cathodecatalyst layer 212 adjacent to the GDL 416 evaporates through the GDL(416) at a rate proportional to the difference between water activity inthe cathode catalyst layer and the partial pressure of the water vaporin the cathode air stream flowing by the cathode GDL, and inverselyproportional to cathode backing thickness. The vapor pressure on theoutside edge of the cathode GDL 416 is determined by the airtemperature, the air flow rate and the relative humidity of the incomingair stream. Thus, very warm dry air from a forced air blower willdrastically increase the rate of water evaporation from the outer faceof the cathode GDL 416.

In an alternative embodiment, as shown in FIG. 7, an AMFC 700 may beoperated with dead-ended anode chamber 218 and a natural convection “airbreathing” cathode configuration, using zero, or minimal supply of waterfrom an external source. In the case of an air breathing cell, watervapor is released through the open face of the cathode by means ofnatural convection, and the rate of water vapor loss will be determinedby the thickness and openness (porosity) of components shielding thecathode catalyst layer from the external environment. Optimization inthis case can be based on a correct trade-off between shielding the AMFCcathode face 712 to prevent excessive water loss and avoiding excessiveblocking of oxygen access to the cathode catalyst. The controlstructural and operation parameters available to achieve such optimizedtrade-off, are the design temperature of the operating cell and theporosity and wetting properties of layers 710, 712 separating thecathode catalyst layer 212 from the outside environment. A cathode gasdiffusion layer 712, 710 of typical dimensions and porosity defines a“cathode limiting current,” corresponding to the maximum rate of oxygentransport by diffusion through the GDL (no convection occurs within thelayer). For a typical GDL used in a fuel cell air electrodes, thisoxygen limiting current is a significant fraction of an ampere/cm². Onecan trade-off the cathode limiting current for better confinement (lowerloss) of water in the cathode and that can be done by either increasingthe thickness of the gas diffusion layer 712, 710 or by decreasing itsdegree of openness. The design considerations of the cathode gasdiffusion layer can be better understood when looking at some equationsdescribing oxygen and water vapor transport rates at some cell current.

Assuming the design current density is J, then the flux of incomingoxygen has to obey: 4F×flux_(O2)=J′ where the flux is in mol/cm² sec andis driven by a partial oxygen pressure differential of maximum 0.2 atmbetween air outside the cathode and the cathode catalyst 212. Therequired flux_(O2) has to satisfy: =J/4F, and thereby determines thedesign gas permeability of the cathode GDL for operation at current J,such permeability implemented by some combination of cathode GDLthickness and porosity.

Water vapor that escapes through the same cathode GDL, is driven by adifferential in water vapor pressure between the inner surface and theouter surface of the GDL and the (same) thickness/porositycharacteristics of the cathode GDL. Therefore, at some inner celltemperature, the rate of water vapor release will fulfill the condition2F×flux_(H2O)=J′ where the pressure gradient determining the water vaporflux through the cathode GDL is: P*_(H2O,Tcell)−P_(H2O, amb), whereP_(H2O, cell) is the vapor pressure in the cathode 210 and P_(H2O, amb),is the ambient water vapor pressure.

It can be seen from the above, that at some cell temperature T_(cell)*and cell current J, the required matching of the fluxes of water vaporfrom the cathode outward and of oxygen from air inward, both fluxesoccurring through the same cathode GDL, can be derived from:

$\begin{matrix}{J_{cell} = {2\; F \times {flux}_{H\; 2O}}} \\{= {2\; {F\left( {1/\delta} \right)}{D_{eff}\left\lbrack {P_{{H\; 2O},{T^{*}{cell}}}^{*} - {P_{{H\; 2O},{amb}}.}} \right\rbrack}}} \\{= {4\; F \times {flux}_{O\; 2}}} \\{= {{k\left\lbrack {4\; {F\left( {1/\delta} \right)}D_{eff}} \right\rbrack}0.2\mspace{14mu} {atm}}}\end{matrix}$

where δ is the thickness of the cathode GDL, D_(eff) is the effectivediffusion coefficient of a gas through it and k is the fraction ofmaximum cathode current (cathode limiting current) at which the celloperates (k=J_(cell)/J_(lim,cath)). 0.2 atm is the maximum oxygenpartial pressure differential achievable between ambient air and acathode catalyst layer of zero oxygen pressure, as will be the case whenthe cathode operates under limiting current conditions. Thus, theoptimum cell temperature will be determined by:

[P* _(H2O,T*cell) −P _(H2O,amb) ]=k 0.4 atm

The latter equation shows that, near k=1, corresponding to operationnear the cathode limiting current, P*_(H2O,T*cell) is near 0.4 atm andthis defines a cell temperature near 70 deg C as a basis for optimized,passive water management. When operating with a GDL of same porosity andhalf the thickness, the same cell current will correspond to 50% of thecathode limiting current (k=1/2) and, consequently, P*_(H2O,T*cell) hasto be near 0.2 atm to lose the correct amount of water and the designcell temperature should consequently be near 50 deg C. Thesecalculations demonstrate that coupling AMFC current design andtemperature design to a an optimized GDL of appropriate thickness andporosity, enables operation in air breathing mode with both sufficientrate of oxygen supply and limited rate of water loss.

The air breathing AMFC 700 includes an adjustable shutter 714. Theadjustable shutter 714 may be manually or processor controlled to movein a such a fashion as to either block off or open up area on the faceof the cathode backing layer 712 in order to fine tune the water loss ofthe cell according to the specific operating conditions. For example, atsome operating conditions of the cell, such as startup, there will below water loss combined with the desire to provide as much oxygen aspossible to the catalyst sites to promote the cathode reactions. Thisstartup mode may be accommodated with the adjustable shutter 714 in anopen state. As the temperature of the cell rises with continuedoperation of the cell due to electrical inefficiency losses andassociated heating of the cell, the water temperature will also risewhich increases the rate of water evaporation and water loss from thecell. During such a running mode, the adjustable shutter 714 may bepartially closed to reduce the effective open area on the face of thecathode backing layer 712. This adjustable shutter 714 may be automatedin a processor implemented control loop with one or more of the controlvariables being parameters such as cell temperature, cell voltage (whichmay give an indication of cell flooding), and current draw of the cell(or stack). The adjustable shutter 714 may offer the AMFC operator adegree of freedom that is similar to the control of a variable forcedair supply from an air blower.

Effective water management in the operating AMFC is required to achievea high hydration level across the thickness dimension of the catalystcoated membrane (CCM) for the complete range of target operationconditions. An AMFC generates water at the fuel/anode side by theprocess:

H₂+2OH⁻=2H₂0+2e

and consumes water at the oxygen or air cathode side of the cell by thecathode process:

2e+(1/2)O₂+H₂O=2OH⁻

According to the latter process, a flux of water proportional to theAMFC current demand must be supplied continuously to the AMFC cathode.For instance, in an AMFC generating a cell current of 1000 mA/cm², thewater flux demanded by the cathode process is 6 mg of water per cm² ofactive area per minute. If the rate of water supply to the AMFC cathodeis any lower, the current will drop and adjust to the more limited fluxof water available. A water consuming electrode process is a uniqueproperty of the AMFC and, consequently, effective water supply to theAMFC cathode is key for maintaining higher AMFC performance.

We disclose here three complementary approaches to water management inthe AMFC, intended to address the significant challenge posed by a waterconsuming cathode. The first approach relies on transport of enoughinternally generated water from the anode side to the cathode sidethrough the cell membrane along the active area of CCM. From theelectrode formulations given above, it can be seen that 50% of the watergenerated at the cell anode is required for the cathode process. If across-the-membrane flux of water along the active area of the AMFC iswell above 50% of the rate of anode water generation, then such mode ofwater transfer to the cathode (“mode 1”) should be sufficient to bothsustain the cathode process and maintain a sufficient hydration levelthrough the cathode. The way to achieve effective hydration by “mode 1”alone, involves, in principle, use of thin membranes, preferably lessthan 50 micrometer thick, yet mechanically robust, in conjunction withGDLs having highly water blocking microporous layers (“MPLs”). Suchcombination guarantees highest water flux across a membrane made ofgiven polymeric material at some given cell current.

Operating the AMFC in “mode 1” hydration exclusively is preferable fromthe perspective of cell structural and operational simplicity. Its useis limited, however, by the magnitude of the cell current at theoperation design point. At steady state, the rate of change in cellwater content is given by:

$\begin{matrix}{{{d\; {m_{{water},{ss}}/{dt}}} = {{k_{1}J_{cell}} - k_{2{\{{Tcell}}}}},_{{{gas}\mspace{11mu} {flow}}\}}{{m_{{water},{ss}} = 0};}} & \left( {1a} \right)\end{matrix}$

i.e., steady state water content, m_(water,ss), in an operating cellwithout any external water supply, is reached when the rate of watergeneration by cell current, given as k₁J_(cell), is equal to the rate ofwater loss, given as k_(2{Tcell, gas flow}) m_(water). From thisequation, the steady state of water in the cell, m_(water,ss), is givenby:

m _(water,ss) =k ₁ J _(cell) /k _(2{Tcell,gas flow})  (1b)

Equation (1b) teaches that the steady state water content will increasewith cell current at some cell temperature and gas flow rate.Consequently, to achieve the maximum possible water content in an AMFCat the beginning of operation with a freshly prepared CCM, a routine ofgradual increase of cell current to the maximum possible value,typically near 1 A/cm², was found by us highly effective for setting ahigh initial cell hydration level. Such routine apparently helps tosignificantly swell the ionomeric membrane and the ionomeric componentsof the electrodes. This cell pretreatment routine facilitates achievingthe high power densities in AMFCs operating without any added liquidelectrolyte.

On the other hand, equation (1b) teaches that, under low cell currentlevels the steady state water content will drop proportionately. Thewater level could consequently become insufficient to secure therequired water flux across the membrane, causing further drop in cellcurrent and further loss in water. To resolve such situation, furthermodes of AMFC cathode hydration, beyond “mode 1,” can be implemented inthe design. We describe here solutions comprising “mode 2” of hydration,involving redirection to the cathode of water that escaped out the backside of GDLs, and “mode 3” of hydration, based on the supply of liquidwater from an external source.

A first option of supplementing water to the AMFC cathode beyond thewater flux across the membrane, is based on trapping and redirectingback to the cathode that part of cell-generated water that escapedthrough the backside of GDLs. On the anode side, the part of theanode-generated water that is not transferred through the membranepenetrates through the GDL into the anode flow field. In operation ofthe anode in open ended mode, this water will leave with the anodeexhaust, whereas in operation in dead-ended mode such water could buildup excessively in the anode compartment behind the GDL. We teach hereeffective ways to redirect water escaping through the anode GDL. In thecase of an open-ended anode, the water leaving out the anode exhaustre-enters a water exchange unit where anode exhaust water content istransferred to large degree to the inlet air stream. In the case of adead-ended anode configuration, we teach enhanced transfer of excessliquid water from the anode side of the membrane to the cathode side bymeans of symmetric frames made of hydrophilic material, placed adjacentthe membrane surface around the GDL. The frame on the anode sideprovides a trap for anode water that had not traversed the membrane andfacilitates passing such water over to the frame facing it on thecathode side of the membrane. Water that escapes through the cathodeexhaust stream, can also be transferred to the incoming air stream bymeans of a second water exchange unit.

Water management utilizing a water source external to the membraneelectrolyte fuel cell (“mode 3” of hydration), typically involveshydration of the CCM by humidified gas feed streams. The effectivenessof this prior-art water supply is limited, because a conflict existsbetween an effective supply of gas and of water through the same cathodebacking layer (gas diffusion layer). In particular, humidification bywater vapor is limited by the level of water vapor content in theincoming gas streams and by the limited flux of water vapor through thegas diffusion layer. Thus, effective supply of water from externalsources should preferably be in the form of direct supply of liquidwater to the cathode side of the AMFC. This can be done, as example, bymeans of an added water channel machined around the gas flow channels onthe cathode side, enabling immediate contact of liquid water with themembrane surface on the cathode side. Such water supply, used inconjunction with a water wicking mesh introduced along the surface areaof the membrane on the cathode side, is one option for direct liquidwater supply to the AMFC cathode. Another option is to have the extraliquid water contained in a reservoir, made as an integral part of thestack structure, with liquid water wicking directly from this reservoirto the cathode side of the membrane surface at a rate which isself-controlled by the cathode hydration level.

In a further aspect of the invention belonging to “mode 3”humidification, transport of water to the cathode side of an AMFC may beaccomplished by establishing side-by-side or parallel flow paths ofoxygen/air and liquid water from the cathode side of the cell. A currentcollection and gas supply plate constructed and arranged for attachmentto the cathode side of an AMFC may be configured with a gas flow fieldand a water channel dedicated to water transport to the cell membrane.The water channel may be configured to transport water to an area of themembrane disposed outside of the periphery of the active area of themembrane to enable water to transport or diffuse laterally along thecross-sectional area of the membrane. The plate may help to createseparate transporting domains that may help to avoid segmentation alongthe cross-sectional area of a cathode gas diffusion layer. Water may besupplied to the channel from a source, e.g., a stationary reservoir,external to an AMFC or internally incorporated with an AMFC or an AMFCstack. Water may be supplied from an external source via a pump.Further, water may be supplied to the channel from water collectingaround an active area of a membrane when the anode side of the AMFC isdead-ended.

Referring again to the “mode 3” of cell humidification, in one aspect ofthe invention, water may be supplied directly to the cathode side of anAMFC from an external water supply to help to mitigate the dependence ofthe water supply solely on fuel cell current. An AMFC designed with oneor more hydrophilic wicks configured for transport of water via wickingaction may help to enhance hydration of an alkaline membrane, and/or mayhelp to enhance the rate of water transport from the anode to thecathode side of the cell. Application of a wick in the form of a meshdirectly to the cathode side of an AMFC membrane may help to enhancehydration of and the amount of water along the cathode surface of themembrane via wicking action whereby the mesh transports water from awater reservoir. The water reservoir may be designed as an externalcomponent to the fuel cell stack, or may be integrated with the designof the stack. The mesh may be in the form of a thin, recastionomer-filled separator between an AMFC membrane and cathode catalystlayer, configured as a porous structure affixed to the membrane with therecast ionomer to help to ensure continuity of the ionic path throughthe membrane to the catalyst layer.

Referring now to FIG. 8, in another aspect of the present invention, afuel cell 1000 includes an anode side 1010 and a cathode side 1020 withan alkaline anion-exchange polymer membrane 1110 disposed between theanode and cathode sides to form an alkaline membrane electrode assembly.The membrane 1110 comprises one or more poly-hydrocarbon materials, andseparates the anode and cathode sides 1010 and 1020 electronically andprovides for the conduction of hydroxide ions. The anode side 1010 ofthe fuel cell 1000 includes an anode catalyst layer 1030 positionedadjacent to the membrane 1110, an anode gas diffusion layer 1050, ananode open electronically-conducting spacer 1070, and an anode bipolarplate 1090. The cathode side 1020 includes a cathode catalyst layer1040, a cathode gas diffusion layer 1060, a cathode openelectronically-conducting spacer 1080, a cathode bipolar plate 1100, thegas inlet 1200 and a hydrophilic micro-fiber mesh 1120 positionedadjacent to the membrane 1110. The cathode side 1020 further includes awater reservoir 1220.

The cathode side 1020 of the fuel cell 1000 is configured to help tosupply water, at a rate independent of the fuel cell current, directlyto an interface of the alkaline membrane 1110 and the cathode side 1020of the fuel cell 1000 to help to ensure a sufficient level of water inthe membrane 1110 and at the cathode side 1020. As shown in FIG. 8, thecathode side 1020 includes the porous hydrophilic micro-fiber mesh 1120disposed between the membrane 1110 and the cathode catalyst layer 1040to serve as such water conduit to the cathode catalyst surface adjacentthe membrane 1110.

Referring to FIG. 9, and with further reference to FIG. 8, the mesh 1120comprises an array of hydrophilic fibers 1130 arranged randomly to forma mesh having a plurality of pores 1150. The mesh of hydrophilic poreshelps to distribute effectively liquid water along the surface of themembrane adjacent the cathode. The pores 1150 are “filled” with orinclude at least one recast ionomer 1170 to help to ensure-an ionic pathfor hydroxide ions. In one embodiment of the invention, the mesh 1120may be formed by applying, e.g., spraying or brushing, a solution of arecast ionomer 1170 to fill the pores 1150 to form a pre-filled mesh1120. The pre-filled mesh 1120 lies across an active area of the fuelcell 1000 between the membrane 1110 and the cathode catalyst layer 1040.In another embodiment of the invention, the mesh 1120 is placed on thecathode surface of the membrane 1110 and a solution of a recast ionomer1170 is applied to the mesh 1120 to fill its pores 1150. Application ofthe recast ionomer 1170 helps to attach the mesh 1120 to the membrane1110. The recast ionomer 1150 also helps to ensure good ionic contactsat two interfaces: between the membrane 1110 and the ionomer-filled mesh1120 and between the mesh 1120 and the cathode catalyst layer 1040. Inone embodiment, the mesh 1120 defines a thickness in a range from about10 microns to about 25 microns, and preferably 15 microns. In oneembodiment, the mesh 1120 comprises thin battery separator materialsbased on fibers of poly(tetra-fluoro ethylene) (“PTFE”) that underwenttreatment to render the surfaces of the fibers hydrophilic.

The fibers comprising the hydrophilic mesh 1120 are constructed of oneor more hydrophilic materials including, but not limited to, cellulose,cotton, or surface modified PTFE, such as, for example, fiber mesh thatis manufactured by and available from W.L. Gore of Elkton, Md.

The position of the mesh 1120 along the membrane 1110 enablesapplication of water directly to the cathode surface of the membrane1110 via a wicking action, while maintaining the continuity of the ionicpath to the catalyst layer from the membrane 1110. In one embodiment,the mesh 1120 is configured and is positioned between the membrane 1110and the cathode catalyst layer 1040, such that, the mesh 1120 helps tosupply water to a substantial area of the interface between the membrane1110 and the cathode catalyst layer 1040.

One edge of the mesh 1120 is in contact, e.g., continuous contact, witha water supply in order to supply water to the cathode side 1020 of themembrane 1110. As shown in FIG. 8, in one embodiment, a lower edge ofthe mesh 1120 extends from the membrane electrode assembly and into thewater reservoir 1220, an appropriately sized, closed compartmentdedicated to providing a water supply to the mesh 1120. The mesh 1120supplies water to the membrane 1110 through a wicking action where themesh 1120 remains in contact with water contained in the water reservoir1220. As the mesh 1120 is positioned over an active area of the membrane1110 and/or a substantial portion of the active area of the membrane1110, the mesh 1120 helps to deliver water via wicking action to asubstantial area of the interface between the membrane 1110 and thecathode catalyst layer 1040 and thereby a substantial area of themembrane 1110.

The reservoir 1220 may contain de-ionized water and may be operativelydisposed external to the fuel cell stack or may be built into theoverall stack design. As shown in FIG. 8, the mesh 1120 may extendthrough gaskets G₁ and G₂ along the cathode side 1020 into the reservoir1220. Conversely, the mesh 1120 may extend into a water reservoir (notshown) built within the cell 1000 structure. In one embodiment, astationary water reservoir is placed adjacent, e.g., the thicknessdimension of, the stack and is equipped with a de-ionized liquid waterinlet for passive water refill; each cell in the stack has a wickextending into it from the water reservoir, thereby achieving multi-cellextension of the scheme described in FIG. 8 for a single cell.

With further reference to FIGS. 8 and 9, assembly of the fuel cell 1000includes, in one embodiment, applying the hydrophilic fiber mesh 1120 tothe cathode side 1020 of the membrane 1110 and brushing or spraying themesh 1120 with a sufficient amount of one or more recast ionomers tosubstantially fill the pores 1150 of the mesh 1120. The cathode catalystlayer 1040 is then applied by spraying or brushing a catalyst ink overthe mesh 1120 and the membrane 1110, wherein, as mentioned above, thecatalyst ink includes a mixture of a solid catalyst and dissolvedionomer. Deposition of the catalyst layer 1040 onto the ionomer-filledmesh 1120 helps to form a continuous ionic path from the membrane 1110to the cathode catalyst layer 1040. The gas diffusion layer 1060 is thenapplied over the catalyst layer 1040, and the cathode bipolar plate 1100is positioned to abut the gas diffusion layer 1060. In one embodiment,the cathode open conducting spacer 1080 is positioned between the gasdiffusion layer 1060 and the bipolar plate 1100. The anode side 1010 ofthe fuel cell 1000 is similarly assembled with the exception the mesh1120 is not included along the anode side 1010.

The edges of the fuel cell 1000 are sealed by pressing a planar edge ofeach bipolar plate 1090 and 1100 over a pair of gaskets G₁ and G₂. Afirst gasket G₁ is positioned directly adjacent the membrane 1110. Asecond gasket G₂ covers and extends over the edges of the gas diffusionlayers 1050 and 1060 and covers and extends over the first gasket G₁. Aportion of the mesh 1120 is slipped through the pair of gaskets G₁ andG₂ to an area external to the fuel cell 1000 where it is received intothe reservoir 1220. The fuel cell 1000 thereby helps to maintain themesh 1120 in continuous contact with water contained within thereservoir 1220.

Referring to FIG. 10, in another aspect, the invention provides a fuelcell 2000 configured to help to facilitate transport of water generatedat the anode side 1010 of the cell 2000 to the cathode side 1020, aswill be particularly important in operation in dead-ended anode mode.The fuel cell 2000 design employs anode and cathode wicks 2040 and 2060to transfer anode-generated water to the cathode side 1020 of themembrane 1110A to help to enhance the rate of water transport to thecathode reaction. In addition, the cross-sectional area of the fuel cell2000 is configured to accommodate an extended area of the membrane 1110Athat defines a highly water-permeable polymeric film configured as aframe 2020 around the membrane 1110A. The wicks 2040 and 2060 permittransport of water from the anode 1010 to the cathode side 1020 of thecell 2000 along the membrane 1110A periphery. In addition, the wicks2020 and 2060 and the frame 2020 significantly enhance the rate of watertransport from the anode 1010 to the cathode side 1020 of the fuel cell2000.

The fuel cell 2000 illustrated in FIG. 10 is constructed with similarcomponents as included in the fuel cell 1000 illustrated in FIG. 8, andlike reference numerals are used to indicate similar components. Theanode side 1010 of the cell 2000 includes the anode wick 2040, the anodecatalyst layer 1030, the anode gas diffusion layer 1050, and the anodebipolar plate 1090 including the gas inlet 1160. The cathode side 1020of the cell 2000 includes the cathode wick 2060, the cathode catalystlayer 1040, the cathode gas diffusion layer 1060, and the cathodebipolar plate 1100 including the gas inlet 1200. The alkaline membrane1110A is disposed between the anode and cathode sides 1010 and 1020 ofthe cell 2000.

As shown in FIG. 10, the water permeable film frame 2020 comprises aframe around the active ion-conducting area of the membrane 1110A. Inone embodiment, the membrane 1110A is constructed to integrate the frame2020, such that, the membrane 1110A and frame 2020 form a unitarycomponent. In another embodiment, the frame 2020 is a separate componentand is positioned adjacent the periphery of the membrane 1110A when thecell 2000 is assembled. The frame 2020 is constructed of one or morepolymer films and/or meshes and has significantly high waterpermeability relative to the water permeability of the membrane 1110A.The polymer films or meshes suitable for constructing the frame 2020include polymers exhibiting high water uptake and high waterpermeability, including, but not limited to, polyalcohol polymers, suchas polyvinyl alcohol (PVA). The polymer films contained in frame 2020 donot require high gas separation properties.

The wicks 2040 and 2060, as mentioned, are positioned along the anodeside 1010 and the cathode side 1020 of the fuel cell 2000. Each wick2040 and 2060 is configured to extend from one edge of the cell 2000 toan opposite edge of the cell 2000, e.g., along the height H₁ of the cell2000, in facing relation to the membrane 1110A. As shown in FIG. 10, thewicks 2040 and 2060 are disposed between the membrane 1110A and therespective catalyst layers 1030 and 1040, and extend beyond the activeion-conducting area of the membrane 1110A. The extended portions of thewicks 2040 and 2060 are disposed adjacent to and are in contact with theframe 2020, such that, the frame 2020 is sandwiched between the anodewick 2040 and the cathode wick 2060 along two sides of the cell 2000,e.g., along the top and the bottom edges of the cell 2000. The wicks2040 and 2060 are further positioned, such that, at least a portion ofthe wicks 2040 and 2060 contacts at least a portion of a surface of themembrane 1110A. A pair of gaskets G₃ and G₄ disposed around a peripheryof the membrane electrode assembly help to seal the wicks 2040 and 2060and the frame 2020. In one embodiment, the seal that the gaskets G₃ andG₄ create substantially seals the frame 2020 from flow of gases suppliedto the fuel cell 2000.

The wicks 2040 and 2060 are constructed of hydrophilic materials, e.g.,an array of hydrophilic fibers and/or hydrophilic material, and areconfigured to transport water to the membrane 1110A via wicking action.In addition, the wicks 2040 are configured to transport water alongtheir surfaces to the water permeable frame 2020. As shown in FIG. 10,the anode wick 2040 receives and transports anode-generated water alongits surface to the frame 2020. The water permeable frame 2020 receivesthe anode-generated water and transmits water there through to thecathode wick 2060. The cathode wick 2060 receives and transports wateralong its surface to an interface between the membrane 1110A and thecathode catalyst layer 1040 thereby enhancing the rate of watertransport to the catalyst side of the membrane 1110A. The wicking actionthat the cathode wick 2060 achieves helps to enhance water transport toa substantial area of the membrane 1110A and the catalyst layer 1040along the cathode side.

Referring to FIGS. 11 and 12, in another aspect, the invention providesa fuel cell 3000 constructed and arranged to help to supply liquid waterdirectly to the surface of the membrane component of the membraneelectrode assembly and just outside the periphery of the active area ofthe membrane. Again, as with FIG. 10, like reference numerals to thefuel cell 1000 are used to indicate similar components. The fuel cell3000 design avoids application of water to the active area of themembrane through the gas diffusion layer by supplying liquid waterdirectly to the membrane surface just outside, e.g., a few millimetersfrom, the periphery of the active area. In this manner, the fuel cell3000 design helps to deliver water to the water-consuming cathode sideof the membrane. As described below, test results suggest that waterpropagates along the lateral dimension of the membrane and helps toenhance the fuel cell 3000 performance, particularly on the cathodeside, to thereby increase the current the cell 3000 produces.

The fuel cell 3000 illustrated in FIG. 11 is constructed with similarcomponents as included in the fuel cells 1000 and 2000 illustrated inFIGS. 8 and 10, where like reference numerals are used to indicatesimilar components, with the differences described below. The anode side1010 of the cell 3000 includes the anode catalyst layer 1030, the anodegas diffusion layer 1050, and the anode bipolar plate 1090 including thegas inlet 1160. The cathode side 1020 of the cell 3000 includes thecathode catalyst layer 1040, the cathode gas diffusion layer 1060, and acurrent collection and gas supply plate 3020 including the gas inlet1200. The current collection and gas supply plate 3020 abuts the gasdiffusion layer 1060. An alkaline membrane 1110B is disposed between theanode and cathode sides 1010 and 1020 of the cell 3000.

FIG. 12 illustrates a schematic end view of the current collection andgas supply plate 3020 that is positioned along the cathode side of thecell 3000. The plate 3020 is constructed and arranged to help to deliveroxygen gas or air to the cathode side 1020 of the membrane electrodeassembly, and is further constructed and arranged to help to supplyliquid water to the periphery 3100 of the active area-of the membrane1110B. The plate 3020 defines a gas flow field 3040, e.g., a singlechannel, serpentine shaped field, along one surface that is configuredand disposed to help to delivery oxygen gas or air to an active area3080 of the membrane 1110B. In one embodiment, the flow field 3040 ispositioned at approximately the center of the plate 3020. When the fuelcell 3000 is assembled, the flow field 3040 is in fluid communicationwith the membrane active area 3080. The plate 3020 further defines awater supply channel 3060, e.g., a single loop, which is configured tohelp to deliver water directly to the membrane 1110B adjacent to andoutside of, e.g., a few millimeters from, the periphery of the activearea 3080. When the cell 3000 is assembled, the water supply channel3060 contacts at least a portion of an area 3100 of the membrane 1110Badjacent to and outside the periphery of the active area 3080 of themembrane 1110B. The configuration and arrangement of the plate 3020 andthe membrane 1110B, help to create a side-by-side, or separate andparallel, transport of oxygen gas/air and water to the membraneelectrode assembly. The flow field 3040 of the plate 3020 helps totransport oxygen/air to the cathode gas diffusion layer 1060 and thesingle loop channel 3060 helps to transport water to the outside area3100 of the membrane 1110B. In this manner, the cell 3000 design createsseparate transporting domains and avoids segmentation along thecross-sectional area of the cathode gas diffusion layer 1060 as requiredto achieve water supply through the gas diffusion layer 1060.

Referring now to FIGS. 13A and 13B, test results indicate that themembrane area 3100 of FIG. 11 permits water to transport effectivelyfrom the peripheral channel 3060 along the lateral dimension-of themembrane 1110B to the active area 3080 to supply water to the activearea 3080 during operation of the cell 3000. Transport of water alongthe lateral dimension of the membrane 1110B is expected to beproportional to the ratio of the cross-sectional area of the membrane1110B to the distance of water transport required along the majorsurface of the membrane. Due to the relatively small or narrowcross-sectional area of the membrane 1110, water transport is expectedto be significantly lower than along the thickness dimension of themembrane 1110B and reliance on lateral diffusion of water through themembrane 1110B material could seem unrealistic. Despite the potentialbarrier to lateral water transport that may result from the relativelysmall cross sectional dimension of the membrane 1110B and thesignificant distance of transport, results shown in FIGS. 13A and 13Breveal the beneficial effect of the supplemental supply of waterdirectly to the cathode side of the membrane 1110B along the area 3100outside the periphery of the active area 3080 of the membrane 1110B.

FIG. 13A shows the test results of the operation of an AMFC with an airsupply to the cathode side of the membrane electron assembly. With theAMFC conductivity depending strongly on water content to the cathodeside, the cathode performance becomes limited in operation on air whenwater is provided to the cell by cell current alone. This is becausecell currents generated in operation on air are significantly lower thanin operation on neat oxygen. In comparison, FIG. 13B shows the testresults of the operation of an AMFC constructed and arranged as shown inFIGS. 11 and 12, indicating direct supply of water to the cathode sideof the membrane electrode assembly by means of a liquid water channelsurrounding the active area helps to increase, e.g., double, the cellcurrent output of the AMFC.

While we believe that these unexpected test results can have differentinterpretations, and the invention does not rely upon any one particularinterpretation of the results, one possible interpretation and/orexplanation is that the area 3100 of the membrane 1110B permits water totransport laterally along an outer surface of the membrane 1110B ratherthan through the cross-sectional area of the membrane 1110B. Under thisinterpretation or explanation, the rate of lateral water transport alonga membrane of an AMFC could enable the higher performance that wereport, and which is illustrated in FIG. 13B.

Transport of water as a thin film along the outside surface of membrane1110B would be facilitated and thereby enhanced if the thin film ofwater deposited from the liquid water channel penetrates under a gasketthat covers the periphery of the membrane 1110B. With further referenceto FIG. 11, in one embodiment, the cell 3000 includes a gasket 3120disposed between the plate 3020 and the membrane electrode assembly. Thegasket 3120 is configured and sized to cover the peripheral edges of theactive area 3080 of the membrane 1110B. The gasket 3120 also includes aslot 3140 that, when the plate 3020 is assembled with the membraneelectrode assembly and the gasket 3120, is disposed over the single loopchannel 3060. The slot 3140 is configured to permit water transport tothe surface of the membrane 1110B. When assembled with the plate 3020and membrane electrode assembly, the gasket 3120 helps to prevent waterfrom permeating or spreading beyond the borders of the single loopchannel 3060 that are defined collectively by the plate 3020 and thegasket 3120. This configuration, however, does not prevent watertransport along the interface of the membrane 1110B—opening up thepossibility of water migration along the interface between the membraneand the cathode catalyst layer 1040.

The latter mode of water distribution can be further improved by addinga water wick, e.g., the mesh 1120, covering the cathode side surface ofthe membrane 1110B, as described with reference to FIG. 8. The waterchannel 3060 is thereby replacing the wicking action external to thecell, as described in FIG. 8, and enhancing the rate of supply of waterfrom an external reservoir to the periphery of the active area andtaking, at the same time, advantage of the subsequent effective lateraldistribution of water by the water-wicking mesh 1120 covering thecathode active area.

Referring to FIG. 14, in another aspect, the invention provides a method4000 of pretreatment of and effective water supply to a fuel cell, suchas any of the fuel cells described above with reference to FIGS. 8-13B,to bring the fuel cell to its full power output and thereafter tomaintain this full power output to the degree it depends on the level ofhydration of the ionomer components in the electrodes and in themembrane. The method 4000 shown in the block flow diagram illustrated inFIG. 14 is exemplary only and not limiting. The method 4000 may bealtered, e.g., by having stages added, removed or rearranged. Ingeneral, the method 4000 includes pretreatment or preconditioning of afuel cell 1000, 2000, and 3000 involving passing current through thecell 1000, 2000, and 3000 for a given period of time and therebyhydrating the cell membrane 1110, 1110A, and 1110B and other ionomercomponents of the cell, to help to achieve a level of humidificationthat helps to increase the degree of ionic conductivity of the membraneand components.

At stage 4020, provide idle assembled fuel cell 1000, 2000, and 3000,e.g., a hydrogen/oxygen alkaline polymer electrolyte membrane fuel cellincluding an ion-conducting membrane 1110, 1110A, and 1110B and placefuel cell in contact with passively-wicked water, e.g., contained inwater tank and kept warm at a preferred or required temperature, whereincell 1000, 2000, and 3000 is at “off” state.

At stage 4040, dial low voltage, e.g., near about 50 mV, across the cell1000, 2000, and 3000, and apply airflow, e.g., high rate of airflow, forgiven period of time, e.g., from about 5 to about 10 minutes, and/oruntil power plateaus, wherein cell 1000, 2000, and 3000 is at “on”state.

At stage 4060, dial load for output power required.

At stages 4080, assess whether voltage is under or in accordance withcell 1000, 2000, and 3000 specifications.

If voltage is in accordance with cell specifications, at stage 4100,determine if voltage remains stable, and if voltage drops with time, atstage 4120, apply controlled pumping of water through water channel andinto cell 1000, 2000, and 3000.

If voltage is under cell specifications, proceed to stage 4120 asdescribed to apply controlled pumping of water through water channel andinto cell 1000, 2000, and 3000.

At stage 4140, determine response of voltage to application of water. Ifcell voltage responds and rises in response to application of water,continue controlled pumping of water at stage 4160.

If cell voltage does not respond to application of water, return to oneor more of stages 4060 thru 4100.

If cell voltage drops after controlled pumping of water, at stage 4180,discontinue water and apply a high rate of airflow for a given period oftime, e.g., about 10 minutes.

Example 1

CCM was prepared by direct spray application of a catalyst ink to thesurface of an anion conducting membrane (Tokuyama). The inks for eithercathode or anode did not contain any platinum and were each based on asolution of an OH— ion conducting ionomer. The membrane was placedduring the spray process on a vacuum table and the CCM was subsequentlypressed together at 100-400 bars. The thickness of the catalyst layerwas under 5 micrometers. Following CCM activation in the cell by passageof current at cell voltage of 50 mV for about 30 minutes, to reach acurrent plateau of 1 A/cm2, the cell generated peak power of 260 mW/cm2in operation on hydrogen and oxygen at cell temperature of 65 degreesCelsius. The power/current curve is shown in FIG. 15 side-by-side withprevious reports for liquid-electrolyte-free AMFCs, using Pt catalysts(University II), non-Pt cathode catalyst (Industry I) and Pt-free cell(University I).

References in FIG. 15 are to the following sources:

-   “University I”-   Shanfu Lu, Jing Pan, Aibin Huang, Lin Zhuang, and Juntao Lu.    “Alkaline polymer electrolyte fuel cells completely free from noble    metal catalysts,” PNAS 2008 105:20611-20614.-   “University II”-   Jin-Soo Park,* Gu-Gon Park, Seok-Hee Park, Young-Gi Yoon, Chang Soo    Kim, Won Yong Lee. “Development of Solid-State Alkaline Electrolytes    for Solid Alkaline Fuel Cells,” Macromol. Symp. 2007, 249-250,    174-182.-   “Industry I”:-   A. Filpi, M. Boccia and H. A. Gasteiger, “Pt-free Cathode Catalyst    Performance in H2/O2 Anion-Exchange Membrane Fuel Cells,” ECS    Transactions 16(2), 1835-1845 (2008).

In describing embodiments of the invention, specific terminology is usedfor the sake of clarity. For purposes of description, each specific termis intended to at least include all technical and functional equivalentsthat operate in a similar manner to accomplish a similar purpose.Additionally, in some instances where a particular embodiment of theinvention includes a plurality of system elements or method steps, thoseelements or steps may be replaced with a single element or step;likewise, a single element or step may be replaced with a plurality ofelements or steps that serve the same purpose. Further, where parametersfor various properties are specified herein for embodiments of theinvention, those parameters can be adjusted or rounded-off toapproximations thereof, within the scope of the invention unlessotherwise specified. Moreover, while this invention has been shown anddescribed with references to particular embodiments thereof, thoseskilled in the art will understand that various substitutions andalterations in form and details may be made therein without departingfrom the scope of the invention; further still, other aspects, functionsand advantages are also within the scope of the invention.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. An alkaline membrane fuel cell comprising: an anode electrode; apolymer membrane electrolyte configured to conduct hydroxyl (OH—) ions,the membrane being in physical contact with the anode electrode on afirst side of the membrane; a cathode electrode in physical contact witha second opposite side of the membrane; wherein the fuel cell isconfigured and designed to transfer water from the anode electrodethrough the membrane to the cathode electrode; further comprising awater exchanger operatively connected with the fuel cell and disposed toexchange water from the cathode electrode exhaust stream to the cathodeelectrode inlet stream.
 2. The device of claim 2 wherein the waterexchanger includes a membrane, the membrane being of high waterpermeability and low gas permeability to enable transport of water fromthe humidified cathode electrode exhaust stream to an inlet stream ofdry air.